The present invention relates to a power converting device having a voltage inverter that can be supplied with direct current power from both a voltage source rectifier, which converts alternating current power generated by an alternating current generator into direct current power, and a direct current power source circuit, and that drives an alternating current load.
To date, as this kind of power converting device, there is known a control device of a hybrid automobile wherein, for example, a three-phase output of an alternating current generator AG rotationally driven by an engine EG is converted to a direct current by a voltage source rectifier IVg, wherein six arms in which switching elements and diodes are connected in inverse parallel are bridge connected, the direct current output is supplied to a voltage source inverter IVm, wherein six arms in which switching elements and diodes are connected in inverse parallel are bridge connected, and converted to an alternating current, and an alternating current motor AM is driven by the alternating current output, as shown in FIG. 13 (for example, refer to JP-A-10-191503).
Also, an electric load device including a DC-DC converter that converts a first direct current voltage output from a direct current power source into a second direct current voltage whose voltage level differs from that of the first direct current voltage, and first and second electric loads driven by the second direct current voltage, has been proposed (for example, refer to JP-A-2004-112883).
A specific configuration of the electric load device has an alternating current motor M1 that drives a drive wheel, and an alternating current generator M2, driven by an engine, that generates alternating current power, as shown in FIG. 14. The alternating current motor M1 is driven by a voltage source inverter IV1, and the alternating current power generated by the alternating current generator M2 is converted into direct current power by a voltage source rectifier IV2. A smoothing capacitor C is connected between a positive bus bar L1 and a negative bus bar L2 connecting the voltage source inverter IV1 and voltage source rectifier IV2. A direct current chopper CV is connected between the positive bus bar L1 and negative bus bar L2 to which the smoothing capacitor C is connected. The direct current chopper CV, as well as raising the voltage of direct current power of a direct current power source B and supplying it between the positive bus bar L1 and negative bus bar L2, lowers the voltage of direct current power input from the voltage source inverter IV1 and voltage source rectifier IV2, and charges the direct current power source B.
Then, the voltage source inverter IV1, voltage source rectifier IV2, and direct current chopper CV are drive controlled by a control device CD.
In the configuration of JP-A-2004-112883. it is disclosed that, in the configuration of FIG. 14, when the direct current chopper CV, or voltage source inverter IV1 or voltage source rectifier IV2, is stopped in an emergency by the control device CD, the voltage source inverter IV1 or voltage source rectifier IV2 is compulsorily stopped, but a detailed description is omitted.
However, although the alternating current motor AM is driven using the output power of the alternating current generator AG and a direct current power source such as a battery in the heretofore known example described in JP-A-10-191503, a tendency is increasing in systems in which the alternating current generator AG is driven by an internal combustion engine to drive the alternating current motor using, as far as possible, only the direct current power source, for the sake of energy conservation and CO2 reduction. For example, in a hybrid automobile in which an internal combustion engine and an electric drive system are used in tandem, a battery or large capacity capacitor is used as a direct current power source, the capacity of the battery or capacitor is increased, enabling the battery or capacitor to be charged from a distribution system too, and a practical application of a plug-in hybrid automobile that travels (electric vehicle (EV) travel: includes braking time) without activating the internal combustion engine during travel in the region of several tens of kilometers is coming near.
In this way, when an electric vehicle (EV) travels for a long time, the output power of the alternating current generator AG is “0”, and normally, the alternating current motor AM continues to be driven or braked by the voltage source inverter, which has the direct current power source as an input, with the rectifier circuit connected to the alternating current generator AG remaining stopped.
Furthermore, in a hybrid automobile called a series-parallel type, one portion of mechanical output torque generated by the internal combustion engine at a time of hybrid travel is transmitted directly to the drive wheel, and at a time of electric vehicle (EV) travel, the output torque transmitted from the internal combustion engine to the drive wheel must also be generated by the alternating current motor AM. For this reason, as the conducting current of the voltage source inverter driving the alternating current motor AM also increases, there is a tendency for loss, such as switching loss, of the voltage source inverter to occur more at a time of electric vehicle (EV) travel than at a time of hybrid travel.
Also, when traveling at low speed, the alternating current voltage necessary in the alternating current motor AM decreases along with a decrease in speed in comparison with the voltage when traveling at medium or high speed, and decreases below the voltage of the battery. In this case, as the voltage of the battery does not decrease, the direct current voltage input into the voltage source inverter is maintained at a high level, and there is also a tendency for switching loss of the switching elements or diodes in the inverter occurring when switching to remain high.
For this reason, it is often the case that inverter loss at low speed travel during electric vehicle (EV) travel is a condition determining a voltage source inverter cooling device, switching frequency, and the like. If it were possible to reduce the input voltage of the voltage source inverter at this kind of low speed travel time, it would be possible to reduce the occurrence of loss of the switching elements and diodes configuring the voltage source inverter, but in the heretofore known example described in JP-A-10-191503. the direct current power source is connected directly to the input side of the voltage source inverter, and it is not possible to lower the inverter input voltage. As a result, the inverter loss during low speed travel increases. Also, there is an unsolved problem in that the configuration of the voltage source inverter increases in size in order to combat this kind of loss.
Also, even in a case in which the direct current chopper circuit CV is provided between the voltage source inverter IV1 and battery B, which is the direct current power source, as in the heretofore known example described in JP-A-2004-112883, although it is possible to carry out a voltage raising action in the direct current chopper circuit CV raising the input voltage of the voltage source inverter IV1 with respect to the battery B, it is not possible to carry out a voltage lowering action lowering the input voltage of the voltage source inverter IV1. For this reason, in the same way as in the heretofore known example described in JP-A-10-191503. it is not possible to reduce the occurrence of inverter loss during low speed travel at a time of electric vehicle (EV) travel, and there is an unsolved problem in that the configuration of the voltage source inverter increases in size.